The nervous system comprises the central and the peripheral nervous system. The central nervous system is composed of the brain and the spinal cord, and the peripheral nervous system consists of all of the other neural elements, namely the nerves and ganglia outside of the brain and spinal cord.
Damage to the nervous system may result from a traumatic injury, such as penetrating trauma or blunt trauma, or a disease or disorder including, but not limited to Alzheimer's disease, multiple sclerosis, Huntington's disease, amyotrophic lateral sclerosis (ALS), diabetic neuropathy, senile dementia, stroke and ischemia.
After spinal cord injury (SCI), spared regions of the central nervous system are spontaneously capable of repairing the damaged pathway, although the process is very limited. Moreover, despite the many promising treatment strategies to improve connections across the damaged spinal cord, the strength of connectivity and functional recovery of the impaired spinal cord are still unsatisfactory. It is well known that spared axons sprout after SCI. See Murray M., Goldberger M. E., Restitution of function and collateral sprouting in the cat spinal cord: the partially hemisected animal, J. Comp. Neurol., 158(1):19-36 (1974); Bareyre F. M., Kerschensteiner M., Raineteau O., Mettenleiter T. C., Weinmann O., Schwab M. E., The injured spinal cord spontaneously forms a new intraspinal circuit in adult rats, Nat. Neurosci. 7:269-77 (2004); Brus-Ramer M., Carmel J. B., Chakrabarty S., Martin J. H., Electrical stimulation of spared corticospinal axons augments connections with ipsilateral spinal motor circuits after injury, J, Neurosci. 27:13793-13901 (2007). But fine-tuning of the process of sprouting of spared axons after SCI as well as synapse stabilization might be dependent on precise pathway-selective activity.
Electrical stimulation of the central and peripheral nervous systems improves neuronal connectivity, and can be employed used to improve functional recovery after neuronal injury. It is an effective method that promotes reactive sprouting through which an increase in the number of functional connections may be possible. Electrical stimulation can also improve functional connections by strengthening the weak existing synapses and/or by promoting synaptogenesis. One of the emerging concepts is that the nervous system contains latent pathways that can be awoken by electrical stimulation or pharmacological manipulation.
The majority of the methods employing electrical stimulation utilize a one-point experimental paradigm in which unipolar or bipolar stimuli are delivered at one point of the sensorimotor pathway. The effectiveness of this stimulation depends on active propagation of an action potential through spared axons. Practically, one-point stimulation would be only effective if the neuronal connections exist and can support active and successful propagation of generated potentials. Therefore, one-point stimulation would be restricted in its efficacy and inclined toward stronger connections.
The loss of neuromuscular activity after SCI leads to inevitable abnormalities that limit the effectiveness of one-point stimulation by blocking excitatory responses from traveling across the sensorimotor pathway. Some of these abnormalities are muscle atrophy and peripheral nerve inexcitability. In addition, changes of the sensorimotor pathway below and above the lesion may involve several different mechanisms; some of them may be maladaptative. This maladaptive function will bias stimuli toward connections with better integrity, further limiting the effectiveness of localized stimulation.
According to the Habbian plasticity principle, physiological processes strengthen synaptic connections when presynaptic activity correlates with postsynaptic firing. See, for example, Hebb D, Organization of Behavior, New York, Wiley (1949). This phenomenon is known as long term potentiation (“LTP”). LTP could be induced by high-frequency presynaptic stimulation or by pairing low-frequency stimulation with postsynaptic depolarization. LTP can also be induced if a pre-synaptic input is activated concurrently with post-synaptic input. In addition, direct current passed through a neuronal pathway can modulate the excitability of that pathway depending on the current polarity and neuronal geometry. In that, anodal stimulation would excite while cathodal stimulation inhibits neuronal activity.
Thus, there is a great desire to improve the effectiveness of electrical stimulation when treating neuromuscular conditions, such as SCI.